With a seemingly constant drip of bad news from around the globe—military conflicts in Eastern Europe and the Middle East, the spread of Ebola, slowing economic growth in Western Europe and many emerging markets—it may feel that the US is the only safe place to invest your money. But not having enough exposure outside the US means not enough diversification, and that can actually mean higher risk and potentially lower returns over the long run.

US and international stocks often perform differently, which is why the result of owning both is usually a better-diversified portfolio. There are many reasons for these performance disparities, such as different economic conditions, different laws and regulations, and different effects from currency movements. Yet there’s another key difference that’s often overlooked: US and international stock markets tend to be made up of very different types of companies.